通过Java的Calendar可以实现很多日期方便的计算，如计算某年某个月有多少天！

Calendar cal = Calendar.getInstance();
cal.set(Calendar.YEAR,2016);
cal.set(Calendar.MONTH, 1 - 1);//Java月份才0开始算
int dateOfMonth = cal.getActualMaximum(Calendar.DATE);

另外附加一点其它的知识

Calendar c = Calendar.getInstance(TimeZone.getTimeZone("GMT+08:00"));    //获取东八区时间
  int year = c.get(Calendar.YEAR);    //获取年
  int month = c.get(Calendar.MONTH) + 1;   //获取月份，0表示1月份
  int day = c.get(Calendar.DAY_OF_MONTH);    //获取当前天数
  int first = c.getActualMinimum(c.DAY_OF_MONTH);    //获取本月最小天数
  int last = c.getActualMaximum(c.DAY_OF_MONTH);    //获取本月最大天数
  int time = c.get(Calendar.HOUR_OF_DAY);       //获取当前小时
  int min = c.get(Calendar.MINUTE);          //获取当前分钟
  int xx = c.get(Calendar.SECOND);          //获取当前秒

Calendar的计算

c.add(Calendar.YEAR, 1);
  c.add(Calendar.MONTH, 1);
  c.add(Calendar.DAY_OF_MONTH, 1);
  int year2 = c.get(Calendar.YEAR);
  int month2 = c.get(Calendar.MONTH) + 1;
  int day2 = c.get(Calendar.DAY_OF_MONTH);
  int firstD = c.getActualMinimum(c.DAY_OF_MONTH);
  int lastD = c.getActualMaximum(c.DAY_OF_MONTH);
  System.out.println("当前时间："+year2 + "-" + month2 + "-"+ day2 + " "+time + ":" + min +":" + xx);
  System.out.println("第一天和最后天：" + firstD +"," + lastD);

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